Climate change determined humanity's global conquest

HUMANS may have conquered the world, but not without a big helping hand from climate change. A model of the last 120,000 years of our history reminds us that, while humans are adaptable, our species is ultimately at the mercy of the climate.

Homo sapiens evolved in Africa about 200,000 years ago, but only left the continent some 70,000 years ago. After that we rapidly went global, colonising Europe and Asia, then Australasia and the Americas. So why did our ancestors linger so long in Africa, and what spurred them to finally move on?

Geneticist Andrea Manica at the University of Cambridge, and his colleagues teamed up with climate modellers to simulate changes in temperature and rainfall across the planet over the past 120,000 years. This allowed them to calculate changes in the vegetation in different regions, which gave an estimate of the amount of food available there.

In turn, the food-supply data drove a model of population and migration, which assumed human migration patterns follow food distribution.

The exercise accurately reproduced the pattern and timings of human expansion out of Africa and across the continents - so far as it is known from the archaeological record - suggesting climate and food supply were key factors.

The model also revealed that climate changes probably had a key role in lifting four major roadblocks to humanity's global takeover (see map).

The first and greatest of these was the Arabian peninsula, a once-impassable desert that trapped humans in Africa for tens of thousands of years. Around 70,000 years ago it became wetter, and coastal areas more fertile. H. sapiens followed the food and initially clustered in what is now Iraq. "Climate is a really good explanation for why they didn't make it out earlier," says Manica.

There are several conceivable routes out across the Arabian peninsula, but the model suggests most humans would have followed the Arabian coast.

From Iraq, one group expanded east and south-east into Indonesia, where they hit a second roadblock: high sea levels made many islands, and ultimately Australia, inaccessible.

Waters fell 60,000 years ago and again 15,000 years later, as successive glaciations trapped more water at the poles. This shortened Asian sea crossings. The climate and vegetation model suggests H. sapiens probably only reached south-east Asia 45,000 to 50,000 years ago, which would rule out a crossing when sea levels fell for the first time.

The model suggests humans reached Siberia by 30,000 years ago, where they were met by a vast ice sheet - the third roadblock. Not until 15,000 years ago did that shrink, allowing them into the Americas.

Back in Europe and Asia, populations faced the final obstacle between them and world domination: local ice sheets, which fluctuated with the ice ages between 55,000 and 15,000 years ago. During warm periods H. sapiens crept north into Scandinavia and northern Asia, but were forced south when the ice encroached again.

"The study fills in many of the links that have only been assumed or guessed at," says Rick Potts of the Smithsonian National Museum of Natural History in Washington DC. "There are inconsistencies," says John Stewart of Bournemouth University in the UK. "But the results are still remarkably good."

To see just how sensitive our species has been to climate, Manica ran the model several times, varying the strength of climate's effect on populations. In parallel, he also modelled the history of genetic variation, and compared that with real data on the genetic make-up of modern populations. Strikingly, he could reproduce known migration timings, and real-world genetic data, only if populations in his model were highly sensitive to the climate.

Even in recent history, societies have declined or collapsed thanks to climate change (New Scientist, 4 August, p 32). "Climate has been a major determinant in the fate of human populations," says Manica, "and that's not going away."

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